iOS - Delegates - iOS (iPhone, iPad) Tutorial

Example for Delegate

Let's believe an item A phone calls an item B to execute an activity. Once the activity is complete, item A should know that B has finished the process and take necessary activity. This is obtained with the help of associates.

The key concepts in the above example are −

A is a delegate object of B.
B will have a reference of A.
A will implement the delegate methods of B.
B will notify A through the delegate methods.

Steps in Creating a Delegate

step 1. First, create a single view application.

step 2. Then select File -> New -> File...

step 3. Then select Objective C Class and click Next.

step 4. Give a name to the class, say, SampleProtocol with subclass as NSObject as shown below.

step 5. Then select create.

step 6. Add a protocol to the SampleProtocol.h file and the updated code is as follows −

#import <Foundation/Foundation.h>
// Protocol definition starts here 
@protocol SampleProtocolDelegate <NSObject>
@required
- (void) processCompleted;
@end
// Protocol Definition ends here
@interface SampleProtocol : NSObject

{
   // Delegate to respond back
   id <SampleProtocolDelegate> _delegate; 

}
@property (nonatomic,strong) id delegate;

-(void)startSampleProcess; // Instance method

@end

step 7. Implement the instance method by updating the SampleProtocol.m file as shown below.


#import "SampleProtocol.h"

@implementation SampleProtocol

-(void)startSampleProcess{
    
    [NSTimer scheduledTimerWithTimeInterval:3.0 target:self.delegate 
 selector:@selector(processCompleted) userInfo:nil repeats:NO];
}
@end

step 8. Add a UILabel in the ViewController.xib by dragging the label from the object library to UIView as shown below.

step 9. Create an IBOutlet for the label and name it as myLabel and update the code as follows to adopt SampleProtocolDelegate in ViewController.h.


#import <UIKit/UIKit.h>
#import "SampleProtocol.h"

@interface ViewController : UIViewController<SampleProtocolDelegate>
{
    IBOutlet UILabel *myLabel;
}
@end

step 10. Implement the delegate method, create object for SampleProtocol and call the startSampleProcess method. The Updated ViewController.m file is as follows −

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    SampleProtocol *sampleProtocol = [[SampleProtocol alloc]init];
    sampleProtocol.delegate = self;
    [myLabel setText:@"Processing..."];
    [sampleProtocol startSampleProcess];
 // Do any additional setup after loading the view, typically from a nib.
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

#pragma mark - Sample protocol delegate
-(void)processCompleted{    
    [myLabel setText:@"Process Completed"];
}


@end

step 11. We will see an outcome as follows. Originally the brand shows "processing...", which gets modified once the assign technique is known as by the SampleProtocol item.